The present invention relates to a display device having a display surface constituted by a number of display elements having only on and off states on which an image having half-tones can be displayed.
In a conventional large scale display device such as a scoreboard in a stadium or in a conventional display device using a flat display panel, display elements such as light bulbs or light emitting diodes are arranged in matrix form. In order to display an image having half-tones with a conventional display device of this type, a technique is employed in which the on or display time period of the individual display elements is controlled in accordance with the amplitude of an image signal. The display element is turned off at the end of the display period. With this system, the light emitting time width of the display elements is made proportional to the amplitude of the image signal.
This system of displaying an image having half-tones by controlling the light emitting time width of each display element has various advantages. However, the system is still disadvantageous in the following point. The display period time Tc during which the display element should be turned on for controlling the display element is represented by the following equation: EQU Tc=TF/[(M-1).times.M]
where TF is the display period, M is the number of tone levels, and N is the total number of display elements. As is clear from the equation, as the number M of tone levels and the number N of display elements increase, the time Tc decreases. Because of systems requirements, it is difficult or impractical to increase M and N.